In a grade crossing (also called a level crossing), gaps in the roadway around the railway tracks are filled in with large concrete, steel or polymeric panels, which rest on wood or concrete ties and are approximately flush with the railheads. As vehicles pass over the crossing, the panels experience significant loading and will deflect downwardly. If a panel is not resting directly on a tie, which happens in situations where tie dimensions are not uniform, for example, that deflection may push the panel sharply against the top surface of the tie, possibly damaging the panel, the tie or both. Repeated uncushioned impacts can eventually cause failure of the panel or tie.
It is therefore preferable to have a protective cushioning means between the panels and the ties, to act as both a shock absorber and to help absorb normal support surface variations. An interface pad is designed to cushion and support the panels and to compensate for any irregularities in the top surface of the tie that might otherwise provide a point of impact when the panels deflect against the ties as vehicles pass over.
It may also be necessary to provide such cushioning between concrete ties and other parts overlying the ties in other railway applications. For example, in a concrete turnout or switch, the rails rest on steel plates that are then connected to the tie. It may be beneficial to provide cushioning between the plates and the ties against any impacts or abrasion. Such interface pads may also be used to cushion trackwork, rails, and/or plates in other railway applications.
U.S. Pat. No. 5,626,289 to Demers Jr. et al. discloses a grade crossing, and briefly refers to “known” interface pads for use between the tie and the panels, without providing any real details of the pads. Similarly, U.S. Pat. No. 4,449,666 to Hales et al. discloses a layer of elastomeric compound to separate the panels of a grade crossing from the ties, to provide flexibility to the panels and allow for adjustment of the height of the panels relative to the rail heads. However, not much more detail about the pad itself is provided.
It is known to simply place an elongated (i.e. covering substantially all of the exposed areas of the tie) interface pad over a tie once the tie is in place under a track. The drawback to this is that it creates an additional step in the installation process, as each pad, known as a conformal pad, must be hand-placed on each tie during installation. Further, because there is no physical connection between the pad and the tie, it is difficult to ensure that the pads stay in place as the ties are installed under the rails, or as panels or other overlying parts are installed on the ties, because these situations generally require at least the ties be moved sideways, such that the parts or panels slide over the top surface of the ties. These sliding motions will tend to scrape any loose pads off the top of the tie.
These drawbacks have been addressed by creating shaped interface pads that interact with multiple surfaces of the tie, providing an improved fit between the tie and the pad. For example, U.S. Pat. No. 6,422,478 to Lucas Jr. and Canadian Patent No. 2281110 to Bruyn both show tie pads that are carefully shaped and engineered not only to provide cushioning pockets within the body of the interface pads, but also to fit over the top chamfers of a concrete tie, keeping the pad in place through friction and cooperation with the tie. Similarly, US Pat. App. No. 2007/0200005 to Corbett Jr. et al. describes several means to attach the pads to the tie, such as wrapping around the chamfered edges of the tie, direct attachment to the panels, and physical abutment with rail attachment hardware, ballast or roadway, but does not discuss mechanically fastening the pads to the ties. The main drawback to these shaped interface pads is that they do not address the issues of increased installation time for placing one or more pads on each tie during installation or of potential loss of the unsecured pads during or after installation.
In order to directly deal with this issue, it is known to glue an elongated interface pad, or one or more smaller interface pads, on top of the tie before shipping the tie to the installation site. However, glued-on interface pads are still highly susceptible to displacement during shipping or at any one of several points in the process involved in constructing grade crossings or turnouts, which wastes money and materials.
More secure methods of connecting interface pads to ties have not gained widespread use, possibly because of the difficulties or costs associated with placing mechanical fasteners, such as bolts, into the top of a concrete tie. Further, simply bolting a flat rubber pad into the top of a tie still may not ensure that the pad stays in place during installation, as the rubber pad can be torn off over the fastener if it is pulled with sufficient force. In addition, placement and number of fasteners may be of concern—a fastener near or around the perimeter of the pad may be more secure and stable than a central fastener, but may then require more fastening points and therefore more time to install enough fasteners to properly secure the pad to the tie.
It is therefore an object of the invention to provide an interface pad, specifically for use in a concrete tie, that overcomes one or more of the foregoing difficulties.
In particular, it is an object of the invention to provide a pre-attached interface pad for a concrete tie that eliminates the need to ship loose conformal pads to the site as well as on-site handling and installation of those conformal pads.
It is a further object of the invention to provide an interface pad that is mechanically attached to a concrete tie, in order to eliminate undesirable movement of the pad during shipping, installation and service.
It is a further object of the invention to provide an interface pad that is manufactured and shaped for secure installation on a concrete tie.
These and other objects of the invention will be appreciated by reference to the summary of the invention and to the detailed description of the preferred embodiment that follow. Note that the objects referred to above are statements of what motivated the invention rather than promises. Not all of the objects are necessarily met by all embodiments of the invention described below or by the invention defined by each of the claims.